Ben,
Good to hear from you. We are using matplotlib v1.0.1_5 on an install from Macports. Hearing that there is simplification logic is very intriguing. Mark On 09/13/11, Benjamin Root <ben.r...@ou.edu> wrote: > On Tue, Sep 13, 2011 at 11:36 AM, Leidner, Mark <mleid...@aer.com> wrote: > > > > > > > > > > > Dear Python/Matplotlib/Ogr Users: > > > > We are recent converts to Python, and are having trouble with some of its > > functionalities. > > We'd like to submit our case for your consideration in hopes to get some > > educated help on the subject. > > > > > > > > The problem: > > When trying to use contour collections generated by contourf, the resulting > > shapefile contains overly simplified contours which poorly approximate the > > underlying field. > > > > To reproduce the problem, we wrote a python script that specifies a 2-d > > analytical shape. This shape has small noise perturbations added, in order > > to simulate natural geophysical fields (wind speed, for example). > > > > > > . > > The shape is being sliced by contourf command, and the resulting collection > > is being plotted as a PDF file (PostScript) and converted to an output > > Shapefile using OGR module. > > > > > > We also wrote several functions, defined inside the script, that take care > > of unpacking and exporting the contour collections as polygon or > > multipolygon shapefile entities thru OGR shapefile methods. > > > > > > > > Two zoomed in views are attached (screenshot_* attachments): (1) a portion > > of the PDF figure, and (2) a visualization of the shapefile data for the > > same area. The PDF figure shows a contour line with fine scale structure > > (the fine structures are the noise we added) while a lack of fine structure > > is seen in the output shapefile. The PDF plot is what we expect. The > > output shapefile geometry is very different from what we would expect. > > > > > > > > > > We can't understand how a call to contourf can produce a plot that looks > > right AND shapefile data (taken from contourf's collections) that appear to > > grossly simplify the geometry. We expect that both the plot and the > > shapefile come from the same contourf function results, yet look totally > > different. > > > > > > > > We'd like to ask whether anyone else encountered limitations regarding the > > complexity of shapefiles written out by python? > > Is this a possible problem with matplotlib.pyplot.contourf.collections > > method? > > > > > > We would appreciate your help very much! > > > > Test script and the resulting shapefile data set are attached. > > > > Thank you! > > > > > > > > > > I am not able to run the test case because I don't have osgeo (also note that > Nio isn't used in the given example). However, I might have a guess as to > what is going on. In mpl, there is path simplication logic to reduce > complexity of the paths. There have been bugs in the past with this logic, > and so it would be valuable to know what version of matplotlib you are using. > > > > This simplification code is probably being activated within the call to > to_polygons(). Which version of matplotlib are you using? > > Ben Root > > > > > >
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